Method of forming a polysilicon film and method of manufacturing a thin film transistor including a polysilicon film

ABSTRACT

In a method of forming a polysilicon film, a thin film transistor including a polysilicon film, and a method of manufacturing a thin film transistor including a polysilicon film, the thin film transistor includes a substrate, a first heat conduction film on the substrate, a second heat conduction film adjacent to the first heat conduction film, the second heat conduction film having a lower thermal conductivity than the first heat conduction film, a polysilicon film on the second heat conduction film and the first heat conduction film adjacent to the second heat conduction film, and a gate stack on the polysilicon film. The second heat conduction film may either be on the first heat conduction film or, alternatively, the first heat conduction film may be non-contiguous and the second heat conduction film may be interposed between portions of the non-contiguous first heat conduction film.

CROSS REFERENCE TO RELATED APPLICATION(S)

This is a divisional application based on application Ser. No.10/980,838, filed Nov. 4, 2004, now U.S. Pat. No. 7,233,022 the entirecontents of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a switching device for use in a flatdisplay panel and a method of manufacturing the same. More particularly,the present invention relates to a method of forming a polysilicon film,a thin film transistor including the polysilicon film, and a method ofmanufacturing the thin film transistor.

2. Description of the Related Art

Thin film transistors are used as switching devices in flat displaypanels such as organic light emitting diodes (OLEDs) or liquid crystaldisplays (LCDs). A channel region of a thin film transistor can becomposed of amorphous silicon or polysilicon.

On one hand, when a channel region of a thin film transistor is composedof amorphous silicon, uniformity of the channel region can be increased.However, due to low mobility of a carrier, it is difficult to attainhigh speed performance. On the other hand, when the channel region of athin film transistor is composed of polysilicon, the mobility of acarrier can be greater than in a channel region composed of amorphoussilicon.

When the channel region of a thin film transistor is composed ofpolysilicon, the polysilicon that constitutes the channel region isformed by crystallizing the amorphous silicon. The method ofcrystallizing the amorphous silicon can be categorized into an excimerlaser method (ELA) and a solid phase crystallization method (SPC). Ofthe two crystallization methods, presently, the ELA method is widelyused, because the ELA method has a lower thermal budget and provides agreater field effect mobility. However, using the conventional ELAcrystallization method, it is difficult to produce crystals grain sizeslarger than 0.5 μm, to obtain uniform grain size and to control aposition on which the crystal is formed.

Accordingly, new methods of crystallization such as a sequential lateralsolidification (SLS) method, an optical phase shift mask (OPSM) method,and a pre-patterned laser beam mask (PLBM) method, have been introduced.However, these methods require a correct substrate control device andrequire multiple pulses of a laser beam. Therefore, there aredifficulties in applying these new methods to the present process forforming thin film transistors.

SUMMARY OF THE INVENTION

The present invention is therefore directed to a method of forming apolysilicon film, a thin film transistor including the polysilicon film,and a method of manufacturing the thin film transistor, whichsubstantially overcome one or more of the problems due to thelimitations and disadvantages of the related art.

It is a feature of an embodiment of the present invention to provide amethod of forming a polysilicon film having larger grain sizes than inconventional films and high uniformity of grain location.

It is another feature of an embodiment of the present invention toprovide a thin film transistor that has a channel region formed ofpolysilicon having larger grain sizes than in conventional thin filmtransistors and high uniformity of grain location.

It is still another feature of an embodiment of the present invention toprovide a thin film transistor having high field effect mobility.

It is yet another feature of an embodiment of the present invention toprovide a thin film transistor that can be formed on a variety ofsubstrates.

It is still yet another feature of an embodiment of the presentinvention to provide a method of manufacturing a thin film transistorthat is able to easily control grain sizes formed in a channel region ofa polysilicon film and provide increased uniformity of grain location.

It is a further feature of an embodiment of the present invention toprovide a simplified method of manufacturing a thin film transistor thatcan is able to omit use of a conventional substrate control device,thereby reducing manufacturing costs.

At least one of the above and other features and advantages of thepresent invention may be realized by providing a method of forming apolysilicon film including forming a lower film including a first heatconduction film and a second heat conduction film, the second heatconduction film having a lower thermal conductivity than the first heatconduction film, forming an amorphous silicon film covering the secondheat conduction film and the first heat conduction film, andcrystallizing the amorphous silicon film.

Forming the lower film may include depositing the second heat conductionfilm on a predetermined region of the first heat conduction film.Alternatively, forming the lower film may include replacing a portion ofthe first heat conduction film with the second heat conduction film.

The method may further include forming a capping film on the amorphoussilicon film after forming the amorphous silicon film. Crystallizing theamorphous silicon film may include irradiating the amorphous siliconfilm with a laser beam having a predetermined energy density through thecapping film. The method may further include removing the capping filmafter irradiating the amorphous silicon film.

Crystallizing the amorphous silicon film may include irradiating theamorphous silicon film with a laser beam having a predetermined energydensity.

At least one of the above and other features and advantages of thepresent invention may be realized by providing a thin film transistorincluding a substrate, a first heat conduction film on the substrate, asecond heat conduction film adjacent to the first heat conduction film,the second heat conduction film having a lower thermal conductivity thanthe first heat conduction film, a polysilicon film on the second heatconduction film and the first heat conduction film adjacent to thesecond heat conduction film, and a gate stack on the polysilicon film.

The second heat conduction film may be on the first heat conductionfilm. Alternatively, the first heat conduction film may benon-contiguous and the second heat conduction film may be interposedbetween portions of the non-contiguous first heat conduction film.

Crystals having grain sizes greater than about 0.5 μm may be formed in aportion of the polysilicon film on the second heat conduction film.

The first heat conduction film may be a film selected from the groupconsisting of an insulating film, a semiconductor film, a metal film,and a silicon carbide (SiC) film. The insulating film may be a filmselected from the group consisting of an aluminum oxide (Al₂O₃) film, astrontium titanium oxide film (SrTiO₃) film, an aluminum nitride (AlN)film, and a silicon carbide (SiC) film. The metal film may be a filmselected from the group consisting of an aluminum (Al) film, a copper(Cu) film, a cobalt (Co) film and a nickel (Ni) film.

The second heat conduction film may be a film selected from the groupconsisting of an organic material film and an inorganic material film.The organic material film may be a film selected from the groupconsisting of a poly acrylonitrite film, a poly methyl methacrylate(PMMA) film, a poly styrene film, a poly vinyl acetate film, a polyvinyl chloride film, a poly ethylene terephthalate (PET) film, and ahybrid silicon polymer film. The inorganic material film may be a filmselected from the group consisting of a silicon oxide (SiO₂) film, amanganese oxide (MnO) film, an air film, and an aerogel film.

The substrate may be a film selected from the group consisting of asemiconductor substrate, a glass substrate, and a plastic substrate.

The thin film transistor may further include a buffer film interposedbetween the substrate and the first heat conduction film.

At least one of the above and other features and advantages of thepresent invention may be realized by providing a method of manufacturinga thin film transistor including forming a first heat conduction film ona substrate, forming a second heat conduction film adjacent to the firstheat conduction film, the second heat conduction film having a lowerthermal conductivity than the first heat conduction film, forming anamorphous silicon film covering the second heat conduction film and thefirst heat conduction film, transforming the amorphous silicon film intoa polysilicon film, and forming a gate stack on the polysilicon filmformed on the second heat conduction film.

Forming the second heat conduction film adjacent to the first heatconduction film may include forming the second heat conduction film on apredetermined region of the first heat conduction film. Alternatively,forming the second heat conduction film adjacent to the first heatconduction film may include replacing a portion of the first heatconduction film with the second heat conduction film, thereby forming anon-contiguous first heat conduction film.

Replacing the portion of the first heat conduction film with the secondheat conduction film may include forming a photo-sensitive patternexposing a portion of the first heat conduction film on the first heatconduction film, removing the exposed portion of the first heatconduction film to form the non-contiguous first heat conduction film,forming the second heat conduction film in the portion where the exposedportion of the first heat conduction film is removed, and removing thephoto-sensitive pattern.

The method may further include forming a buffer film between thesubstrate and the first heat conduction film. The method may furtherinclude forming a capping film on the amorphous silicon film.

Transforming the amorphous silicon film into the polysilicon film mayinclude irradiating the amorphous silicon film with a single pulse of alaser beam having a predetermined energy density. Transforming theamorphous silicon film into the polysilicon film may include irradiatingthe amorphous silicon film with a single pulse of a laser beam having apredetermined energy density through the capping film.

The method may further include removing the capping film afterirradiating the amorphous silicon film.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent to those of ordinary skill in the art bydescribing in detail exemplary embodiments thereof with reference to theattached drawings in which:

FIG. 1 illustrates a cross-sectional view of a thin film transistoraccording to a first embodiment of the present invention;

FIG. 2 illustrates a cross-sectional view of a thin film transistoraccording to a second embodiment of the present invention;

FIGS. 3 though 6 illustrate cross-sectional views of stages in a methodof manufacturing the thin film transistor shown in FIG. 1;

FIGS. 7 through 13 illustrate cross-sectional views of stages in amethod of manufacturing the thin film transistor shown in FIG. 2;

FIGS. 14 through 19 are scanning electron microscope (SEM) images ofcrystals formed in channel regions of polysilicon films formed accordingto an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Korean Patent Application No. 2003-77763, filed on Nov. 4, 2003, in theKorean Intellectual Property Office, and entitled: “Method of FormingPolysilicon Film, Thin Film Transistor Comprising Polysilicon Film andMethod of Manufacturing the Same,” is incorporated by reference hereinin its entirety.

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which exemplary embodimentsof the invention are shown. The invention may, however, be embodied indifferent forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. In thefigures, the dimensions of films, layers and regions are exaggerated forclarity of illustration. It will also be understood that when a layer isreferred to as being “on” another layer or substrate, it can be directlyon the other layer or substrate, or intervening layers may also bepresent. Further, it will be understood that when a layer is referred toas being “under” another layer, it can be directly under, and one ormore intervening layers may also be present. In addition, it will alsobe understood that when a layer is referred to as being “between” twolayers, it can be the only layer between the two layers, or one or moreintervening layers may also be present. Like reference numerals refer tolike elements throughout.

First Embodiment

A thin film transistor according to the first embodiment of the presentinvention will now be described.

FIG. 1 illustrates a cross-sectional view of a thin film transistoraccording to the first embodiment of the present invention.

Referring to FIG. 1, a buffer film 12 with a thickness of about 6000 Åand a first heat conduction film 14 having high thermal conductivity anda thickness of about 1000 Å are sequentially formed on a substrate 10.The substrate 10 may be formed of a semiconductor, glass, or plastic.The buffer film 12 blocks impurities included in the substrate 10 frommigrating from the substrate 10 into elements formed above the bufferfilm 12 during formation of a thin film transistor on the buffer film12. The first heat conduction film 14 may be one selected from the groupconsisting of an insulating film, a semiconductor film, and a metalfilm. When the first heat conduction film 14 is an insulating film, thefirst heat conduction film 14 may be an oxide film, such as an aluminumoxide film (Al₂O₃) or a strontium titanium oxide film (SrTiO₃), or anitride film, such as an aluminum nitride film (AlN). When the firstheat conduction film 14 is a metal film, the first heat conduction film14 may be one selected from the group consisting of an aluminum (Al)film, a copper (Cu) film, a cobalt (Co) film, and a nickel (Ni) film.The first heat conduction film 14 may also be a different material film,such as a silicon carbide (SiC) film.

A second heat conduction film 16 is disposed on a predetermined portionof the first heat conduction film 14. The second heat conduction film 16is formed of a material having a lower thermal conductivity than thefirst heat conduction film 14. The second heat conduction film 16 may bean organic material film or an inorganic material film. When the secondheat conduction film 16 is an organic material film, the second heatconduction film 16 may be a poly acrylonitrite film, a poly methylmethacrylate (PMMA) film, a poly styrene film, a poly vinyl acetatefilm, a poly vinyl chloride film, a poly ethylene terephthalate (PET)film or a hybrid silicon polymer film. When the second heat conductionfilm 16 is an inorganic material film, it may be one selected from thegroup consisting of a silicon oxide (SiO₂) film, a manganese oxide (MnO)film, an air film, and an aerogel film.

A polysilicon film 18 covering the second heat conduction film 16 isformed on the first heat conduction film 14. The polysilicon film 18 issymmetrical about the second heat conduction film 16. The polysiliconfilm 18 is divided into a source region 18 s disposed on one side of thesecond heat conduction film 16, a drain region 18 d disposed on theopposite side of the second heat conduction film 16, and a channelregion 18 c disposed on an upper surface of the second heat conductionfilm 16. At this time, grain sizes in the channel region 18 c of thepolysilicon film 18 may be greater than about 0.5 μm, as illustrated inFIGS. 14 through 19. Accordingly, much greater field effect mobilitythan was conventionally possible can be obtained in the channel region18 c.

A gate insulating film 20 and a gate electrode 22 are sequentiallyformed on the channel region 18 c. The first heat conduction film 14,the polysilicon film 18, the gate electrode 22, and the gate insulatingfilm 20 are covered by an interlayer insulating film 24. A first contacthole h1 that exposes the source region 18 s and a second contact hole h2that exposes the drain region 18 d are formed in the interlayerinsulating film 24. A first electrode 26 that fills the first contacthole h1 and a second electrode 28 that fills the second contact hole h2are formed on the interlayer insulating film 24.

Second Embodiment

A thin film transistor according to the second embodiment of the presentinvention will now be described. Descriptions of elements substantiallysimilar to those described in connection with the first embodiment willnot be repeated.

FIG. 2 illustrates a cross-sectional view of a thin film transistoraccording to the second embodiment of the present invention.

Referring to FIG. 2, the buffer film 12 is on the substrate 10. Anon-contiguous first heat conduction film 14′ is disposed on the bufferfilm 12. A second heat conduction film 16′ with a thickness equal to athickness of the non-contiguous first heat conduction film 14′ isinterposed between portions of the first heat conduction film 14′. Apolysilicon film 18′ covering the whole surface of the second heatconduction film 16′ is deposited on the non-contiguous first heatconduction film 14′. The polysilicon film 18′, in a region where itcontacts the second heat conduction film 16′, i.e., the channel region18 c, is formed of polysilicon having much greater grain sizes than inthe conventional art.

The gate insulating film 20 and the gate electrode 22 are sequentiallyformed on the channel region 18 c. The interlayer insulating film 24covers the non-contiguous first heat conduction film 14′, the gateelectrode 22, the gate insulating film 20, and the polysilicon film 18′.The first and second contact holes h1 and h2 are formed in theinterlayer insulating film 24. The source region 18 s of the polysiliconfilm 18′ is exposed by the first contact hole h1, and the drain region18 d is exposed by the second contact hole h2. The first electrode 26filling the first contact hole h1 and the second electrode 28 fillingthe second contact hole h2 are deposited on the interlayer insulatingfilm 24.

As an alternative to the above-described first and second embodiments, athin film transistor (TFT) such as a bottom TFT in which the gateelectrode 22 is formed under the channel region 18 c, as opposed to onthe channel region 18 c as in the thin film transistors according to thefirst and second embodiments of the present invention, may be formed. Inthe case of the bottom TFT, the gate electrode 22 may be disposedbetween the first heat conduction film 14 or 14′ and the second heatconduction film 16 or 16′.

First Embodiment

A method of manufacturing a thin film transistor according to the firstembodiment of the present invention will now be described.

FIGS. 3 though 6 illustrate cross-sectional views of stages in a methodof manufacturing the thin film transistor shown in FIG. 1.

Referring to FIG. 3, the buffer film 12 and the first heat conductionfilm 14 are sequentially formed on the substrate 10. The substrate 10may be formed of semiconductor, glass, or plastic. The buffer film 12may be formed of a silicon oxide film. In this case, the buffer film 12may be formed to a thickness of about 6000 Å. The buffer film 12 blocksimpurities from migrating from the substrate 10 into elements above thebuffer film 12 in a subsequent process. The first heat conduction film14 may be formed to a thickness of about 1000 Å by a reactivesputtering. The first heat conduction film 14 may be an insulating film,a semiconductor film, or a metal film, but in the present embodiment, itis exemplarily an insulating film. When the first heat conduction film14 is an insulating film, the first heat conduction film 14 may beformed with an oxide film such as an Al₂O₃ film or a SrTiO₃ film, or anitride film such as an AlN film, or a SiC film. When the first heatconduction film 14 is formed with a metal film, it may be one selectedfrom the group consisting of an Al film, a Cu film, a Co film, and a Nifilm.

The second heat conduction film 16 is formed on a predetermined regionof the first heat conduction film 14. The first and second heatconduction films 14 and 16 will be a lower film formed of an amorphoussilicon film. The second heat conduction film 16 is formed by performinga photolithography after depositing a material film on the upper surfaceof the first heat conduction film 14. At this time, the material filmmay be formed to a thickness of about 500 Å using an Inductively CoupledPlasma-Chemical Vapor Deposition (ICP-CVD) apparatus. The second heatconduction film 16 defines the channel region since the channel regionis formed on the second heat conduction film 16 in a subsequent process.

After forming the second heat conduction film 16, an amorphous siliconfilm 17 with a thickness of about 500 Å is formed on the second heatconduction film 16 and the first heat conduction film 14. At this time,the amorphous silicon film 17 may be formed using sputtering or plasmaenhanced CVD apparatus. A capping film 30 may be formed on the amorphoussilicon film 17. The capping film 30, however, is not necessary, and maybe omitted. The capping film 30 may be a silicon oxide film formed to athickness of about 1000 Å using the ICP-CVD apparatus. After forming thecapping film 30, a laser beam L having an energy density sufficient toreach the amorphous silicon film 17 through the capping film 30 isincident on an upper surface of the capping film 30. The laser beam Lhas an energy density of between about 240 to 280 mJ/cm². For example,the laser beam L may be generated using an XeCl excimer laser thatgenerates a short pulse excimer laser beam having a 10 ns period, orusing a Nd-YAG laser. A single pulse of the laser beam L may be used.

When the laser beam L is incident on the capping film 30, heat isgenerated from the entire area of the amorphous silicon film 17. As aresult, the amorphous silicon film 17 is transformed into a polysiliconfilm. At this time, heat generated in a portion of the amorphous siliconfilm 17 deposited on the first heat conduction film 14, which has highthermal conductivity, rapidly dissipates in directions as indicated byfirst, second, and fourth arrows A1, A2, and A4. Heat generated in aportion of the amorphous silicon film 17 deposited on the second heatconduction film 16, which has low thermal conductivity, however, slowlydissipates due to the adiabatic effect of the second heat conductionfilm 16. For this reason, nucleation is formed at both edges of theamorphous silicon film 17 deposited on the second heat conduction film16. The nucleation grows from the both sides of the amorphous siliconfilm 17 toward the inside of the amorphous silicon film 17, as indicatedby third arrows A3, and develops grains. The grain growth begins at bothedges of the amorphous silicon film 17 and continues until meeting onthe second heat conduction film 16.

Referring to FIG. 4, the amorphous silicon film 17 is transformed into apolysilicon film 18 through the above described process, and much largergrain sizes (greater than about 0.5 μm) than were conventionallypossible are formed in the channel region 18 c, that is, the polysiliconfilm 18 deposited on the second heat conduction film 16. Because theformation of the polysilicon film 18 is performed at a low temperatureof about 25-150° C., the substrate 10 may be formed of a silicon wafer,a metal foil, glass, or plastic.

After transforming the amorphous silicon film 17 into the polysiliconfilm 18, the capping film 30, if used, is removed.

The gate insulating film 20 and the gate electrode 22 are sequentiallyformed on the polysilicon film 18 deposited on the second heatconduction film 16. Subsequently, the interlayer insulating film 24 isformed on the gate electrode 22, the gate insulating film 20, thepolysilicon film 18 and the first heat conduction film 14. A surface ofthe interlayer insulating film 24 is then planarized. A photo-sensitivepattern PR1 is formed on the interlayer insulating film 24. Thephoto-sensitive pattern PR1 exposes regions of the interlayer insulatingfilm 24 corresponding to the source region 18 s and the drain region 18d of the polysilicon film 18.

Referring to FIG. 5, after forming the photo-sensitive pattern PR1, theexposed portions of the interlayer insulating film 24 are etched usingthe photo-sensitive pattern PR1 as an etch mask. The etching iscontinued until the source and drain regions 18 s and 18 d of thepolysilicon film 18 are exposed. Thus, the first contact hole h1 throughwhich the source region 18 s is exposed and the second contact hole h2through which the drain region 18 d is exposed are formed in theinterlayer insulating film 24. After completion of the etching, thephoto-sensitive pattern PR1 is removed.

Referring to FIG. 6, after depositing a metal film (not shown) thatfills the first contact hole h1 and the second contact hole h2 on theinterlayer insulating film 24, the metal film is patterned to form thefirst electrode 26 that contacts the source region 18 s and the secondelectrode 28 that contacts the drain region 18 d using photolithography.

Second Embodiment

A method of manufacturing a thin film transistor according to the secondembodiment of the present invention will now be described. Descriptionsof aspects, steps and elements substantially similar to those describedin connection with the first embodiment will not be repeated.

FIGS. 7 through 13 illustrate cross-sectional views of stages in amethod of manufacturing the thin film transistor shown in FIG. 2.

Referring to FIG. 7, the buffer film 12 and the first heat conductionfilm 14′ are sequentially formed on the substrate 10. A photo-sensitivepattern PR2 that exposes a predetermined portion of the first heatconduction film 14′ is formed on the first heat conduction film 14′. Theexposed portion of the first heat conduction film 14′ is etched usingthe photo-sensitive pattern PR2 as an etch mask. The etching iscontinued until the buffer film 12 is exposed, thereby making the firstconduction film 14′ non-contiguous.

Referring to FIG. 8, after etching, the second heat conduction film 16′is formed on the exposed portion of the buffer film 12. At this time, itis preferable that the second heat conduction film 16′ is formed to athickness equal to that of the non-contiguous first heat conduction film14′ to completely fill the removed portion of the first heat conductionfilm 14′. However, the second heat conduction film 16′ formed within theremoved portion of the first heat conduction film 14′ can be formedthicker than the first heat conduction film 14′ provided that the secondheat conduction film 16′ formed on the photo-sensitive pattern PR2 doesnot contact the second heat conduction film 16′ formed on the removedportion of the first heat conduction film 14′. After forming the secondheat conduction film 16′ where the first heat conduction film 14′ wasremoved, the photo-sensitive pattern PR2 is ashed and stripped off.While removing the photo-sensitive pattern PR2, the second heatconduction film 16′ formed on the photo-sensitive pattern PR2 is alsoremoved. Referring to FIG. 9, the non-contiguous first heat conductionfilm 14′ and the second heat conduction film 16′ have the same planeafter removing the photo-sensitive pattern PR2. The first and secondheat conduction films 14′ and 16′ will be a lower film on which anamorphous material film will be formed.

Referring to FIG. 10, the amorphous silicon film 17 is formed on thefirst and second heat conduction films 14′ and 16′, and the capping film30 is formed on the amorphous silicon film 17. Next, the laser beam L isincident on the capping film 30, using a laser as described inconnection with the first embodiment of the present invention.

Due to the laser beam L irradiation, the amorphous silicon film 17 istransformed into the polysilicon film 18′ the channel region 18 c ofwhich has grain sizes greater than about 0.5 μm, which is larger thanthe size of conventional grains. The source region 18 s and the drainregion 18 d together with the channel region 18 c are defined in thepolysilicon film 18′.

Arrows A1, A2, A3 and A4 of FIG. 10 are described above in connectionwith the first embodiment, and a discussion thereof will not berepeated.

The transformation of the amorphous silicon film 17 into the polysiliconfilm 18′ by the laser beam L has been described above. After the laserbeam L irradiation, the capping film 30, if used, is removed.

Referring to FIG. 11, a gate stack GS is formed on the polysilicon film18′ formed on the second heat conduction film 16′. The gate stack GSincludes the gate insulating film 20 and the gate electrode 22, whichare sequentially formed. The gate stack GS may alternatively be formedunder the channel region 18 c of the polysilicon film 18′.

Next, the interlayer insulating film 24 is formed on the gate stack GS,the polysilicon film 18′ and the first heat conduction film 14′. Aphoto-sensitive pattern PR3 is formed on the interlayer insulating film24. The photo-sensitive pattern PR3 exposes portions of the interlayerinsulating film 24 corresponding to the source and drain regions 18 sand 18 d. Referring to FIG. 12, after forming the photo-sensitivepattern PR3, the interlayer insulating film 24 is etched until thesource and drain regions 18 s and 18 d of the polysilicon film 18′ areexposed, using the photo-sensitive pattern PR3 as an etch mask. Thus,the first contact hole h1 through which the source region 18 s isexposed and the second contact hole h2 through which the drain region 18d is exposed are formed in the interlayer insulating film 24.Subsequently, the first electrode 26 that contacts the source region 18s and the second electrode 28 that contacts the drain region 18 d areformed on the interlayer insulating film 24, as shown in FIG. 13.

Experiments measuring the grain size of the polysilicon film 18 or 18′used as the channel region in the TFT, and a relationship between theenergy density of the laser beam L incident on the amorphous siliconfilm 17 and the grain size formed in the channel region 18 c of thepolysilicon film 18 or 18′ were performed. For these experiments, aplurality of amorphous silicon films, which are equivalent to theamorphous silicon films 17 described above, were prepared, and cappingfilms were formed on the prepared amorphous silicon films. Further, asingle pulse from an excimer laser having a different energy density wasincident on each of the prepared amorphous silicon films through thecapping film.

FIGS. 14 through 19 are SEM images of channel regions of the pluralityof polysilicon films formed in the above-described experiments. In FIGS.14 through 19, reference character NA represents an edge of the channelregion, reference number 100 represents nucleation formed randomly atthe edge of the channel region, and reference character SL represents adiameter of the grain formed in the channel region.

FIG. 14 is a SEM image of a channel region of a polysilicon film formedby irradiating the amorphous silicon film with an excimer laser beamhaving an energy density of 240 mJ/cm². In this case, the grain sizeformed in the channel region is approximately 0.8 μm.

FIG. 15 is a SEM image of a channel region of a polysilicon film formedby irradiating the amorphous silicon film with an excimer laser beamhaving an energy density of 260 mJ/cm². In this case, the grain sizeformed in the channel region is approximately 2.3 μm.

FIG. 16 is a SEM image of a channel region of a polysilicon film formedby irradiating the amorphous silicon film with an excimer laser beamhaving an energy density of 280 mJ/cm². In this case, the grain sizeformed in the channel region is approximately 1.3 μm.

FIG. 17 is a SEM image of a channel region of a polysilicon film formedby irradiating the amorphous silicon film with an excimer laser beamhaving an energy density of 300 mJ/cm². In this case, the grain sizeformed in the channel region is approximately 0.5 μm.

FIG. 18 is a SEM image of a channel region of a polysilicon film formedby irradiating the amorphous silicon film with an excimer laser beamhaving an energy density of 320 mJ/cm². In this case, the grain sizeformed in the channel region is approximately 0.5 μm.

FIG. 19 is a SEM image of a channel region of a polysilicon film formedby irradiating the amorphous silicon film with an excimer laser beamhaving an energy density of 340 mJ/cm². In this case, the grain sizeformed in the channel region is approximately 0.5 μm.

From FIGS. 14 through 19, it is seen that a desirable energy density ofthe excimer laser beam incident on the amorphous silicon film forforming the polysilicon film is between about 240 to 280 mJ/cm², whichis consistent with methods described in the embodiments of the presentinvention.

A polysilicon film, a thin film transistor including a polysilicon film,and a method of manufacturing a thin film transistor according to anembodiment of the present invention may exhibit one or more of thefollowing advantages. First, since the grain size (greater than about0.5 μm) of the polysilicon film, which may be used as the channelregion, is much larger than in the conventional art, a high field effectmobility can be obtained.

Second, since the process for crystallizing the amorphous silicon filmmay be performed at room temperature using a laser beam, the grain sizeformed in the channel region of the polysilicon film can easily becontrolled and the grain can uniformly be formed.

Third, a thin film transistor may be formed on a variety of substratesbecause it is formed in a low temperature process using a laser beam.

Fourth, there is no restriction to use a conventional method of forminga thin film transistor process because a conventional excimer laser or asolid state Nd-YAG laser may be used.

Fifth, a desired size of grains may be obtained using a single pulse ofa laser, not multiple pulses, and therefore, a conventional substratecontrol device is unnecessary, thereby reducing manufacturing cost.

Exemplary embodiments of the present invention have been disclosedherein and, although specific terms are employed, they are used and areto be interpreted in a generic and descriptive sense only and not forpurpose of limitation. For example, one skilled in the art could applythe technique of transforming an amorphous silicon film into apolysilicon film having a larger grain size not only to manufacturing athin film transistor, as described herein, but also to manufacturingother devices that use a polysilicon film. Accordingly, it will beunderstood by those of ordinary skill in the art that various changes inform and details may be made without departing from the spirit and scopeof the present invention as set forth in the following claims.

1. A method of manufacturing a thin film transistor, comprising: Forminga first heat conduction film on a substrate; Forming a second heatconduction film adjacent to the first heat conduction film, the secondheat conduction film having a lower thermal conductivity than the firstheat conduction film; forming a polysilicon film in direct contact withat least one surface of the second heat conduction film and in directcontact with at least one surface of the first heat conduction film,wherein the polysilicon film is formed on the first and second heatconduction films and includes a source region and a drain region; andforming a gate stack on the polysilicon film formed on the second heatconduction film.
 2. The method as claimed in claim 1, wherein formingthe second heat conduction film adjacent to the first heat conductionfilm comprises forming the second heat conduction film on apredetermined region of the first heat conduction film.
 3. The method asclaimed in claim 1, wherein forming the second heat conduction filmadjacent to the first heat conduction film comprises replacing a portionof the first heat conduction film with the second heat conduction film,thereby forming a noncontiguous first heat conduction film.
 4. Themethod as claimed in claim 3, wherein replacing the portion of the firstheat conduction film with the second heat conduction film comprises:forming a photo-sensitive pattern exposing a portion of the first heatconduction film on the first heat conduction film; removing the exposedportion of the first heat conduction film to form the noncontiguousfirst heat conduction film; forming the second heat conduction film inthe portion where the exposed portion of the first heat conduction filmis removed; and removing the photo-sensitive pattern.
 5. The method asclaimed in claim 1, wherein the substrate is a film selected from thegroup consisting of a semiconductor substrate, a glass substrate, and aplastic substrate.
 6. The method as claimed in claim 1, furthercomprising forming a buffer film between the substrate and the firstheat conduction film.
 7. The method as claimed in claim 1, wherein thefirst heat conduction film is a film selected from the group consistingof an insulating film, a semiconductor film, a metal film, and a siliconcarbide (SiC) film.
 8. The method as claimed in claim 1, wherein thesecond heat conduction film is a film selected from the group consistingof an organic material film and an inorganic material film.
 9. Themethod as claimed in claim 1, wherein forming the polysilicon filmcomprises: forming an amorphous silicon film covering the second heatconduction film and the first heat conduction film; and transforming theamorphous silicon film into a polysilicon film.
 10. The method asclaimed in claim 9, further comprising forming a capping film on theamorphous silicon film.
 11. The method as claimed in claim 10, whereintransforming the amorphous silicon film into the polysilicon filmcomprises irradiating the amorphous silicon film with a single pulse ofa laser beam having a predetermined energy density through the cappingfilm.
 12. The method as claimed in claim 11, further comprising removingthe capping film after irradiating the amorphous silicon film.
 13. Themethod as claimed in claim 9, wherein transforming the amorphous siliconfilm into the polysilicon film comprises irradiating the amorphoussilicon film with a single pulse of a laser beam having a predeterminedenergy density.
 14. A method of forming a polysilicon film, comprising:forming a lower film including a first heat conduction film and a secondheat conduction film, the second heat conduction film having a lowerthermal conductivity than the first heat conduction film; forming anamorphous silicon film in direct contact with at least one surface ofthe second heat conduction film and in direct contact with at least onesurface of the first heat conduction film, wherein the amorphous siliconfilm is formed on an upper surface of the first and second heatconduction films and includes a source region and a drain region; andcrystallizing the amorphous silicon film.
 15. The method as claimed inclaim 14, wherein forming the lower film comprises depositing the secondheat conduction film on a predetermined region of the first heatconduction film.
 16. The method as claimed in claim 14, wherein formingthe lower film comprises replacing a portion of the first heatconduction film with the second heat conduction film.
 17. The method asclaimed in claim 14, further comprising forming a capping film on theamorphous silicon film after forming the amorphous silicon film.
 18. Themethod as claimed in claim 17, wherein crystallizing the amorphoussilicon film comprises irradiating the amorphous silicon film with alaser beam having a predetermined energy density through the cappingfilm.
 19. The method as claimed in claim 18, further comprising removingthe capping film after irradiating the amorphous silicon film.
 20. Themethod as claimed in claim 14, wherein crystallizing the amorphoussilicon film comprises irradiating the amorphous silicon film with alaser beam having a predetermined energy density.